The influence of hard substratum reflection and calibration profiles on in situ fluorescence measurements of benthic microalgal biomass.

Measuring chlorophyll-a fluorescence is a commonly used method to determine microphytobenthic biomass expressed as chlorophyll-a per square centimetre. However, this in situ method is affected by reflection from the substratum which triggers an additional fluorescence signal within the microphytobenthic biofilm. Depending on the colour and texture of the natural substratum, this effect can lead to a considerable overestimation of microphytobenthic biomass. The results cannot be corrected for this effect by performing an auto-zero measurement, since the overestimation is not caused by an offset of the fluorometer. This article describes a substratum-specific correction procedure using a 700 nm signal to eliminate this effect by quantifying the fluorescence signal as a result of the reflection. An empirical relationship between the 700 nm signal and the additional fluorescence is used to calculate a correction factor for the reflective properties of the substratum. The factor is determined and applied during each biomass measurement, thereby making an additional calibration step for each individual type of substratum superfluous. This new method improves the reliability of the results significantly without increasing the time necessary to perform the measurements and without complicating the measurement procedure.

Establishment of an Alert Level Framework for cyanobacteria in drinking water resources by using the Algae Online Analyser for monitoring cyanobacterial chlorophyll a.

The Algae Online Analyser (AOA) fluorometer simultaneously distinguishes four different phytoplankton groups by their specific fluorescence spectra and thus allows for real-time in-situ chlorophyll a measurements per algal group. This AOA was used for monitoring cyanobacterial chlorophyll a in the drinking water at the Bronislawow Bay abstraction point in Sulejow Reservoir (Poland). The main goal of this research was to develop an early warning method for the detection of cyanobacterial biovolume in the source water, in order to establish an Alert Level Framework for the drinking water abstraction point in Sulejow Reservoir. A positive correlation between cyanobacterial biovolume, as determined by conventional methods, and cyanobacterial chlorophyll a, as measured by the AOA, was found (p<0.05). The results of this study were used to determine threshold values for the Alert Level Framework, based on cyanobacterial chlorophyll a concentrations in the source water of Sulejow Reservoir. The presented threshold values are determined specifically for this abstraction point, but the principles can be applied to other locations.